Toymods Car Club Inc: Tech & Conversions => Trouble with car amplifier

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beena
Occasional Poster

Location:
Melbourne
Registered:
June 2002

Re: Trouble with car amplifier	Sun, 23 June 2002 13:29

There seem to be some SERIOUS misconceptions about electronics and electric circuits here. I will attempt to explain this in a simple and understandable way. Voltage: Simply put this is what is 'supplied' by a source. In this case it is the battery (or alternator). It is roughly 12V (the varying speed of the alternator and varying loads on the cars electrical system means it usually fluctuates between about 11.5 and 14.5V) So you 'give' 12V to the amplifier (the load). Current: This is determined by what the 'load' wants. In this case the amplifier. it will 'draw' whatever current it needs regardless of what size cable it is connected with. So a bigger wire WILL NOT deliver 'too much' current. If a wire that is too small it will simply heat up if a large current is drawn - this is how a light bulb works and how a fuse works. A light bulb has a large current go through a small wire which makes it very hot and therfore gives light! A fuse is similar, when a current is TOO large the fuse gets too hot and melts (blows) whichs breaks the circuit. That is why it is imperative that a power cable is properly fused, if it shorts a HUGE current will flow and it will literally light up! This causes fires and all sorts of other nastyness. Power: Is simply a relationship between voltage and current. Power = Voltage x Current (P=VI) So if an amplifier 'uses' 600W (VERY different to it's audio output due to efficiency etc, and will change depending on volume etc) and the source delivers 12V it will draw a current of 50Amps - regardless of cable size. If this exceeds the fuse's rating it will blow. Resistance: Resistance will not affect current. It will however drop voltage. If you measure the voltage across your battery and again across your amp you will find that it has dropped - sometimes by as much as 1V! This is because the power cable has a 'resistance'. A smaller cable will have greater resistance and a larger cable less - hence less of a voltage drop! Therefore it is ALWAYS better to have a larger cable for less resistance. A small cable will cause a greater drop in voltage which can damage equipment. Most amps will shut down in an 'under-voltage' situation. How does this all relate? The fuse on the amp will blow for one reason: Too greater current flows through it. Why this has happened is what we want to know. NEVER replace a fuse without knowing what blew it. In this case it could be a couple of things: A. The fuse is too small for the amp. Unlikely as this usually occurs while playing music and here it occurs straight away. B. There is a short in the system. A short happens when the source (battery) is presented with little or no resistance (Load) like when the power cable touches the body and the circuit is through the power cable and back to the battery through the body. This will allow a HUGE current to flow and should blow the fuse. The main fuse should blow in this situation. Check ALL the power cable for any possible short. Also check polarity (that +ve is in +ve and vice versa). If it was crackling when you wire up the speakers it means that the amp was powered up, eg ignition was on - not good! However if the speaker wire was grounded (touched the body) it may have blown the amp fuse when the amp was on. So check all that too. This sort of thing is really hard to diagnose remotely, especially without knowing what amp, fuse ratings, install etc. My best advice is to VERY CAREFULLY (and with the battery disconnected) check ALL wiring and go from there. Also check the amp manuals for the fuse rating you should be using. What everyone else is saying is true, if you don't really know what you're doing take it back to the guys that installed it for you and let them check it out. Better safe than sorry! The last thing you want is to return to your car to find it on fire due to faulty audio wiring... (trust me i've seen it happen) As i said i tried to explain all of this in a simple way so some things aren't completely technically correct but should be ok in this situation. If anyone wants to know more, or in more detail, just ask. I can explain it all the way down to coulombs, potential difference and electron flow if you like! Cheers, Robin. robinjp@mac.com [Report message to a moderator]

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